Monovalent‐Divalent Cation‐Exchange Equilibria in Soils in Relation to Organic Matter and Type of Clay

Cation‐exchange equilibria at pH 7 were measured using ion pairs of Na‐Ca, Li‐Ca and Li‐Mg on each of 10 soils. Data on Li‐Mg and Na‐Ca ion pairs were nearly the same and data on Li‐Ca were highly correlated with exchange equilibria data on both Na‐Ca and Li‐Mg. Two soils containing largely amorphous clay minerals showed a higher preference for Li than soils containing dominantly crystalline clay minerals. Destruction of the organic matter in two soils increased the ratio of Na to Ca adsorption to a much greater extent than would be predicted by the decrease in measured surface charge density. At a sodium‐adsorption ratio (SAR) of 100 (mmole per liter) 1/2 , organic‐matter free soils containing dominantly montmorillonite and illite clays had ratios of Na to Ca adsorption of 1.66 to 1.94 (excluding one soil, 1.66 to 1.78). Two soils with dominantly amorphous minerals had ratios of 0.92 and 1.10, and one soil with a mixture of kaolinite, illite, and amorphous clays had a ratio of 1.35.